The present invention relates to a polypropylene resin composition excellent in mechanical properties, particularly rigidity and stretchability, and a biaxially stretched film comprising the polypropylene resin composition.
Since the biaxially stretched polypropylene film is excellent in optical properties such as transparency and luster, heat-resistance, chemical-resistance and electrical properties as well as in mechanical properties such as tensile strength and rigidity, it is broadly used for packaging and preparing electric components such as capacitor, electric cable, etc. However, in the case of using therefor there are cases that the above-mentioned properties have been sufficiently satisfied and accordingly, the enlargement of the actual use of the biaxially stretched polypropylene film has been restricted. Particularly, the biaxially stretched polypropylene film is inferior to cellophane and polyester film in rigidity (so-called nerve) and accordingly, it is difficultly applicable to automatic packaging in overlap-packaging and twist-packaging. In addition, due to the insufficient rigidity thereof, the biaxially stretched polypropylene film is apt to be crumpled when it is processed into capacitor film resulting in the weak point of reducing the processability.
The rigidity of polypropylene is greatly affected by the draw ratio and the structure thereof. The term "polypropylene" means propylene homopolymer and random copolymer of propylene and a small amount of ethylene. Since the propylene homopolymer is superior to the random copolymer in rigidity, the stretched propylene homopolymer is generally used in the case of necessitating rigidity. As the method for further improving the original rigidity of the propylene homopolymer, a method (1) of using propylene homopolymer of higher than 98.0% of isotactic index (hereinafter referred to as I.I.) has been proposed (refer to Japanese Patent Application Laid-Open (KOKAI) No. 57-103819).
In addition, as the method for improving the stretchability of the propylene homopolymer, a method (2) for using the random copolymer, for instance, the copolymer of propylene and ethylene prepared by polymerizing propylene in the presence of a catalyst comprising titanium trichloride and an organic aluminum compound, prepared by reducing titanium tetrachloride by an organic aluminum compound and further activating the thus reduced substance, while adding a small amount of ethylene thereto in the course of polymerizing propylene, has been proposed, the content of ethylene units in the copolymer being 0.1 to 1.0% by weight (refer to Japanese Patent Application Laid-Open (KOKAI) No. 56-32512).
However, although according to the method (1) the rigidity of the propylene homopolymer is improved by raising I.I. thereof, the stretchability of the thus obtained propylene homopolymer is reduced still less and accordingly, it is more difficult to stretch the thus obtained propylene homopolymer at a high draw ratio, and as a result, it is impossible to obtain stretched polypropylene films so much excellent in rigidity.
On the other hand, although according to the method (2) the stretchability of the random copolymer is somewhat improved, I.I. of the thus produced random copolymer is highly reduced by the copolymerization with an extremely small amount of ethylene and accordingly, only a stretched polypropylene film of limpness is obtained
As has been shown above, although the methods for improving either the rigidity or the stretchability of "polypropylene" have been proposed, the polypropylene excellent in both rigidity and stretchability has not been developed.
In consideration of the conventional techniques, the present inventors have studied the methods for improving both the rigidity and the stretchability of the polypropylene, and as a result, it has been found that by combining a small amount of a specified hydrocarbon polymer with a specified polypropylene, the stretchability of the thus prepared polypropylene resin composition is greatly improved while retaining the intrinsic rigidity of the polypropylene, and based on the finding, the present invention has been attained.